Climate stress may undermine male spiders’ romantic gift giving

Courtship dazzle in spiders can lose some zing in uncertain climates. Males in places with hard-to-predict rain and temperatures devolve into suitors who woo mostly with cheap, useless gifts.

Researchers have described gift giving in courtship in only 15 or 20 of the world’s more than 50,000 known species of spiders, says evolutionary biologist Maria José Albo of Universidad La República in Montevideo, Uruguay (SN: 7/26/16). Since 2015, she and her lab have focused on spiders that flirt mostly in evenings and nights among the rocks and pebbles of rivers of Uruguay and southern Brazil.

When the brown male Paratrechalea ornata senses a female to woo, he picks some object to spin silk around for courtship presentation. Males’ bodies are only about the size of peppercorns suspended on legs, so the presents are “very, very small,” Albo says.

The best of these gifts are nuggets of fresh insect meat a female can feed on while a male inserts sperm. The more time she spends opening and eating her gift, the more sperm he can deliver. That abundance could help as sperm in her reproductive tract from various males compete for dad-hood.

So it was surprising when, while monitoring spider silk-wrapped gifts at six locations in Uruguay and Brazil, Albo and colleagues found some pitiful snacks on offer.

The first Albo opened was … a seed. That’s useless for a spider. Their mouthparts work more like milkshake straws, but for meat-shakes. Over years of monitoring courtship gifts, Albo and colleagues have found a bunch of equally unsnackable presents: broken shards of insect exoskeletons, bits of plant stems and so on.

At two study sites, more than half the gifts analyzed were inedible silk-wrapped trash, Albo and colleagues report in The American Naturalist November issue. What these two trash-gift zones for courtship had in common, the researchers say, is low predictability in rainfall and temperature.

In places with more surprises and stresses, perhaps with more floods or out-of-whack insect-prey cycles, spiders have many other challenges. So maybe what’s wrapped up inside competitive gifts just isn’t as important anymore. Gifting persists, but sexual selection based on what’s inside has “relaxed,” as biologists put it. Among the many implications, Albo says, the weakening of a courtship signal “shows how variation in climate, can affect something very essential — which is reproduction.”

Also, it’s “definitely possible” that the mercurial weather could change not just contents of males’ gifts but the important silk wrapping, says behavioral ecologist Michelle Beyer, of Ludwig-Maximilians-Universität München in Germany. She wasn’t part of the South American study but works with Europe’s best-known gift-giving counterpart, the nursery web spider Pisaura mirabilis. Gift-giving males of species on both continents dope their silk with compounds that in some way enhance fatherhood.

“It’s definitely possible,” she says, that rain or high temperatures could change the composition of the silk as well as the length of time the males’ doping compounds last on it. Plus, the silk road of information runs both ways. Research is already under way on the female silk of the European species, she says, to see if spinnings by heat-stressed females maintain their usual appeal to males.